Arrangement for automatic and continuous measuring of the noise factor of an electric device



Aug. 25, 1959 H. G. MoLLFoRs ARRANGEMENT FOR AUTOMATIC AND CONTINUOUS MEASURING OF THE NOISE FACTOR OF AN ELECTRIC DEVICE Flled Sept 24. 1954 t. Kwik A MP1. /f/EA J PL 6E GEA/ERA TOR gils! w.

United States Perm Of Application September 24, 1954, Serial No. 458,090

Claims priority, application Sweden November 25, 1953 2 claims. (cl. 324457) The present invention refers to an arrangement for automatic and continuous measuring of the noise factor of an electric device. It comprises a noise voltage source, controlled by a pulse generator, which noise voltage source is intended to supply a noise voltage in the shape of a pulse train to the input side of the electric device, and a measuring device for measuring the quotient between the output voltage of the electric device during time intervals when pulses appear and during time iutervals when pulse spaces appear in said pulse train.

An arrangement according to the invention can be said to be characterized thereby, that said pulse generator is connected on one hand to the input side of the voltage source via a pulse selector for suppressing every second pulse of a pulse train coming from the pulse generator, on the other hand to the input side of a mixing device to which also the output side of the electric device is connected.

In general, the noise factor (ligure) of an electrical device is defined in the following way:

The noise factor (F) equals the ratio:

The input signal/ noise power ratio The output signal/noise power ratio or in symbols :Sin/Nin Sont/Noot When measuring noise figure it is most convenient to use noise instead of a signal for Sm. This noise is generated in a noise source. The noise output of a noise source is defined as the ratio Noise added to the input of electrical device Noise existing at the input of electrical device or Sm/N,Jn also termed excess noise.

In the prior art teachings F is proportional to Sin/Nin. This means that F is obtained by the use of a variable noise source. Sin/Niu is measured and yields F. Therefore, it is not possible (with 3 db attenuation) to measure the noise factor F of an electrical device, the noise factor of which is greater than the maximum output of the noise source.

In contradistinction to the above, in the present invention the noise factor may be Written Patented Aug. 25, 1959 gas discharge tube), thequotient Peut/Nom, being measured Iin aquotient meter (cross current meter).

It is here possible to measure a noise factor F of au electrical device, the noise factor of which is greater than the noise output of the noise source.

The invention will be described more closely in connection with the accompanying drawing, which in principle shows a block scheme of an arrangement according to the invention.

Inthe drawing number l indicates the electric device, in the following called object to be tested, for example an amplifier, the noise factor of which one wishes to measure. A pulse generator 2 is arranged to generate a pulse train a-l-b and is connected to a pulse selector 3, which has two output sides and is arranged to divide y the pulse train a-l-b into two separate pulse trains a and b, each of them containing half the number of pulses and with three times broader pulse spaces between consecutive pulses compared with the original pulse train. One output side of the pulse selector 3 is connected to an amplifier 4 which in its turn is connected to a noise voltage source 5. The output side of the noise voltage source is connected to the input side of the object 1 to be tested, the output side of which via a gate 6 and a detector 7 is connected to a measuring device 8-9-10, which consists of two amplifiers, 8 and 9 respectively, and a quotient meter 10. The output side of the pulse. generator 2 is also connected to the mixing device 6..

The amplifiers 8 and 9 are both on their input side con nected to the output side of the detector 7 and on their` output side to a separate coil in the quotient meter 10.. The amplifier 8 is connected to the second output side of the pulse selector 3 and is controlled by the pulse train b.. The amplifier 9 is connected to the first output side of the pulse selector 3 and is controlled by the pulse train a.

The arrangement now described is intended to function in the following way.

The pulse train a-i-b, generated in the pulse generator 2, Ais brought to the pulse selector 3, Where it is divided into two pulse trains, a and b.

The pulse train a controls on one hand the noise voltage source S (via the amplifier 4), on the other hand the amplifier 9. The pulse train b controls the amplifier 8. The noise voltage source 5 emits to the object 1 to be tested a noise voltage c in the shape of a pulse train with the same pulse Width and pulse repeating frequency as the pulse train a. In the output side of the object 1 to be ltested appears a composed noise voltage d, the shape of which is depending on the noise voltage c as Well as the `internal noise voltage of the object 1 to be tested. This composed noise voltage and the pulse train a-i-b gi've rise to a composed noise voltage pulse train e in the gate 6, which pulse train can be said to be composed by a noise Voltage pulse train generated from noise voltage source 5 and the object 1 to be tested, and a noise voltage pulse train generated from noise voltage solely from the object 1 to be tested. Asvvill appear from, the Wave forms on the drawing, the long parts d1 and e1 of the pulse trains d yand e correspond to a-pulses and the short parts d2 and e2 correspond to b-pulses. 'Ihe two noise voltage pulse trains have `the same pulse repeating frequency and equally large pulse spaces. Between two consecutive noise voltage pulses of the composed noise voltage pulse train there is `a pulse space, the width of which is the same as the Width of the pulse spaces of the pulse train a-l-b. The composed noise voltage pulse train is fed to the detector 7 which has quadratic characteristic. With the help of the pulse controlled amplifiers 8 and 9 every second direct current impulse from the output side of the detector 7 is fed to one of the coils of the quotient meter 10 `and every second to the other coil.

On this account the quotient meter 10 can be calibrated direct for indicating the noise factor of the object 1v to be tested.

leansf 1- A system; for the eptomatie and oontinuops, urina. of the noise. factor of. eleotreel. device comprising in combinati.oul a Ynoise voltage Source. connected. to.

the input of the ele'etiieal, device..'and ffiiialpteflV .to Supply. a noise Vvoltage in the. Shape of a' pulse train, a pulse genoraiof oomseeted to Said noise voltage souree.- to. Qontrol the-seme, e measuring device formeasuring the quotient between the output signal of the electrical device during time intervals when pulsos-appear and during i11- fervels when pulse Spaces appear in the pulse' train, e-

pulSe Soleotor interconnected between .thesaid pulse generator and said noise voltage source to suppress every second pulse of the pulse train output from said pulse generator, vand a gating device connected to the output of said pulse generator andv'to the output of s aid electrical device, the said measuring device connected .t0 the output 20 of said gating device.V

2. A system according to ,laim 1, including two,r ampliers'nterconnected parallel between the said gating device and 'said'measui'ing' device, the said amplifiers References Cited in the le of thispatent `STA'IES PATENTS.

2.6204-3 De?. 2a 19.52

2,691,098 seiprve :W M l 1 oct. 5, 1954 FOREIGN PATENTS $695,219.- .Great Britain May.v 16, .1945

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